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Dependency Injection - Pico Container

PicoContainer is a lightweight dependency injection (DI) container for Java applications. DI is a technique used in software engineering to achieve loose coupling between objects in an application. Loose coupling means that objects are independent of one another and can be easily replaced or modified without affecting the rest of the application. PicoContainer allows developers to define the dependencies of an object in a declarative way, making it easier to manage complex object graphs. The container takes care of instantiating and injecting the required objects, allowing the developer to focus on writing business logic instead of managing object creation and initialization. One of the key features of PicoContainer is its simplicity. It is easy to learn and use, with a small codebase and a straightforward API. It also supports multiple configuration formats, such as XML, annotations, and programmatic configuration. PicoContainer is often used in small to medium-sized applications, where a full-fledged DI framework may be overkill. However, it can also be used in larger applications as a lightweight alternative to more complex frameworks. Overall, PicoContainer is a useful tool for managing object dependencies in Java applications, helping developers write more modular and maintainable code. How pico container can be used in applications PicoContainer can be used in Java applications to manage object dependencies and promote loose coupling between objects. Here are some steps to use PicoContainer in an application:

  • Define your objects and their dependencies: Identify the objects in your application and the dependencies between them. You can define your objects using Java classes, interfaces, or annotations.

  • Configure PicoContainer: Create a PicoContainer instance and configure it with the objects and their dependencies. You can configure PicoContainer using XML files, Java code, or annotations.

  • Retrieve objects from PicoContainer: Once you have configured PicoContainer, you can retrieve objects from it. PicoContainer will automatically create and inject any dependencies required by the object.

  • Use objects in your application: Once you have retrieved objects from PicoContainer, you can use them in your application as you normally would.


Different types of Dependency Injection supported by PicoContainer

PicoContainer supports multiple ways to specify the injection of dependencies into components. Constructor injection (listed first) is the recommended idiom for PicoContainer. Other types leverage fields and methods. Variations of the method types, can follow a naming convention or be marked with an annotation. Dependencies for those could be populated one by one, or all in one method call. Indeed components could be populated with combinations of Constructor, Method and Field Injection.

Regular Types Description

Constructor”:

constructor-injection.html Where a the constructor of a class is used to

pass dependencies into it


“Setter”:setter-injection.html Multiple Setter methods on the class are used to pass dependencies in.


Annotated Method Methods are marked with an annotation to indicate that they should be used for injecting dependencies.



Field Fields are injected into for dependencies

Annotated Field Fields are marked with an annotation to indicate that they should be injected into


Named Field Fields of a certain name should be injected into


Named Method If method names match other component names, injection happens .


Typed Field Fields of a certain type should be injected into


Irregular Types Description

Adapting Adapting Injection finds which types of injection are pertinent to a component and builds an injector that fits it precisely. This is the default type of injection for DefaultPicoContainer .


Multi Multiple Injection points in order: Constructor, Annotated Methods and then finally Annotated Fields .


Composite As Multi, but an open list of Injection types that makes most sense for your components.


Here's an example of how PicoContainer can be used in a Java application



// Define your objects and their dependencies
public interface DataService {
    List<String> getData();
}

public class DatabaseDataService implements DataService {
    public List<String> getData() {
        // retrieve data from database
    }
}

public class ReportGenerator {
    private DataService dataService;
    
    public ReportGenerator(DataService dataService) {
        this.dataService = dataService;
    }
    
    public String generateReport() {
        List<String> data = dataService.getData();
        // generate report using data
    }
}

// Configure PicoContainer
DefaultPicoContainer container = new DefaultPicoContainer();
container.addComponent(DataService.class, DatabaseDataService.class);
container.addComponent(ReportGenerator.class);

// Retrieve objects from PicoContainer
ReportGenerator reportGenerator = container.getComponent(ReportGenerator.class);

// Use objects in your application
String report = reportGenerator.generateReport();









In this example, we defined two objects: DataService and ReportGenerator. ReportGenerator depends on DataService to retrieve data. We then configured PicoContainer to create and manage these objects. Finally, we retrieved ReportGenerator from PicoContainer and used it to generate a report.


PicoContainer can add behaviors to components automatically during instantiation.

Behaviors are delivered by passing a BehaviorFactory reference into the PicoContainer instance on construction. For the most part BehaviorFactories are completely stateless, and the Behavior instances they make are not. General use DefaultPicoContainer can take a behavior factory instance in its constructor:

pico = new DefaultPicoContainer(new BehaviorClass()); pico.addComponent(Foo.class); // other components added Foo foo = pico.getcomponent(Foo.class) // Foo instance will be affected by an additional behavior. Or it can take behaviors that wrap each other: pico = new DefaultPicoContainer(new BehaviorClass().wrap(new AnotherBehaviorClass())); pico.addComponent(Foo.class); // other components added Foo foo = pico.getcomponent(Foo.class) // Foo instance will be affected by two additional behaviors. Behaviors can be signaled by properties per component:

pico = new DefaultPicoContainer(); pico.as(SOME_BEHAVIOR).addComponent(Foo.class); // the behavior has a property marking it, and the default component factory understands that property // other components added Foo foo = pico.getcomponent(Foo.class) // Foo instance will be affected by an additional behavior.

You can build a container with the applicable property: pico = new PicoBuilder().withXXXBehavior().build(); pico.addComponent(Foo.class); // other components added Foo foo = pico.getcomponent(Foo.class) // Foo instance will be affected by an additional behavior.

Behaviors can be chained together by PicoBuilder: import static org.picocontainer.behaviors.Behaviors.xxxxxxx; import static org.picocontainer.behaviors.Behaviors.yyyyyyy; ... pico = new PicoBuilder().withBehaviors(xxxxxxx(), yyyyyyy()).build(); pico.addComponent(Foo.class); // other components added Foo foo = pico.getcomponent(Foo.class) // Foo instance will be affected by two additional behaviors.

Specific Behaviors Caching Caching is where PicoContainer ensures that the same component instance is returned for second and subsequent getComponent() invocations. See the Caching behavior page. Other Dependency Injection Containers call this Singleton behavior. Implementation Hiding Hiding Implementations is where PicoContainer ensures that the component that implements a abstract type can only be used as that abstract type by components that depend on it. Casting back to the implementation is vetoed. See the Hiding behavior page. Thread Safety Synchronizing and Locking are variations on where PicoContainer ensures that components can used in a thread-safe way. See the Thread Safety behavior page. Interception AOP Style Interception is where PicoContainer can invoke functionality before or after component method execution.


Conclusion


Hope you all enjoyed reading about it . Happy Learning!





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